In recent years, accompanied with miniaturization of electronic equipments, a secondary battery having high capacity has been required. Attention has been paid particularly to a non-aqueous solvent secondary battery having higher energy density as compared with nickel-cadmium and nickel-metal hydride batteries. As a negative electrode material thereof, a metal, graphite, etc. have heretofore been studied. However, in a metal electrode, there is a problem that when charging and discharging are repeated, a metal in a solvent is precipitated in a dendrite form and both electrodes are short-circuited finally. Further, in graphite, metal ions can go in and out between layers thereof so that there is no problem of short circuit, but there are problems that it decomposes a propylene carbonate type electrolyte and also charging and discharging cycle characteristics are bad in an ethylene carbonate type electrolyte. On the other hand, use of a carbonaceous material having a multiphase structure has been also studied. This is based on a thought that advantages (high capacity and small irreversible capacity) and disadvantages (decomposition of a propylene carbonate type electrolyte) of a carbonaceous material having high crystallinity and advantages (excellent stability in an electrolyte) and disadvantages (small capacity and large irreversible capacity) of a carbonaceous material having low crystallinity are combined to compensate the disadvantages while maintaining both of the advantages. For example, in Japanese Provisional Patent Publication No. 370662/1992 which the present inventors have proposed previously, there has been disclosed use of carbonaceous material particles which comprise a nucleus comprising a highly graphitized carbonaceous material and a surface layer comprising a lowly graphitized carbonaceous material. However, in said conventional technique, it has been considered that from limitation in preparation, if it is not a relatively thick surface layer, a uniform layer cannot be obtained. Thus, it has been considered that a composite carbonaceous material having a relatively thick surface layer and clearly comprising plural phases is preferred as shown in, for example, an example in which the ratio of a nucleus to a surface layer is 50 parts by weight:50 parts by weight (Example 1), an example of 53:47 (Example 2) and an example of 65:35 (Example 3) as specific examples thereof.
However, the present inventors have studied electrode materials having various physical properties intensively and consequently found that contrary to a conventional technical concept that a composite carbonaceous material having a relatively thick surface layer and clearly comprising plural layers is preferred as a secondary battery electrode material, by using an electrode material in which to a graphite-like carbonaceous material is attached a carbonized material of an organic substance in an extremely minute amount (thin) to give a specific range of a carbon residue, there can be obtained unexpectedly a non-aqueous solvent secondary battery having extremely good electric characteristics such as high electric capacity and lowly suppressed retention and also having high safety to an electrolyte as compared with graphite alone and a conventional composite carbonaceous material clearly comprising plural phases, to accomplish the present invention.
Further, they have found a specific preparation process for obtaining such a specific electrode material and found that by such a method, an electrode material having more uniform characteristics can be prepared stably and efficiently, to complete the present invention.